In communication systems, there is always a challenge to obtain good performance and capacity for a given communications protocol, i.e., using parameters which are most suitable for f the physical environment in which the communication system is deployed.
It is envisioned that there will soon be a large amount of connected devices, i.e., devices that are able to communicate with other devices. It is further envisioned that a large fraction of the connections will be between a sensor device and a gateway device. In general terms, sensor data, such as temperature, humidity, velocity, etc., typically contains very little data and needs to be transmitted from the sensor device to the gateway device relatively seldom at irregular intervals. The gateway device could for example be a mobile phone, a laptop computer, or a fixed mounted access point (AP). The backhaul for the gateway device may either be a cellular network based on radio access technologies of the 3rd Generation Partnership Project (3GPP) such as LTE (Long Term Evolution) or WCDMA (Wideband Code Division Multiple Access) or it could be a wired network.
As the amount of data to be transmitted increases, the time for the actual data starts to be of more important and the time for setting up the connection will be more important.
In case small amounts of data are transmitted from the sensor device at irregular intervals, this data should be transmitted in a power efficient way to the gateway device. One reason for this is that although the time needed for the actual data transfer is very small, the time it takes to resynchronize the two devices (i.e., the sensor device and the gateway device) often take a relatively long time and therefore accounts for the larger part of the power consumption. In Bluetooth, for instance, when a device wants to connect to another device to which is previously been connected it enters the page state and start transmitting a page train. This paging process may have a duration for more than a second before the connection is established. If the actual data to be transmitted is only one packet, this transmission, including an acknowledgement of the packet, takes 1.25 ms.
When Bluetooth was introduced, the highest data rate was 1 Mbps gross rate using binary modulation in a bandwidth of 1 MHz. In Bluetooth Low Energy (BLE) the signal transmitted over the air (i.e. over the radio interface) is very similar to what is the case for Bluetooth legacy. BLE is part of the Bluetooth 4.0 specification, and more details about BLE can for instance be found in the book by Robin Heydon: “Bluetooth Low Energy, The developer's handbook”. For BLE the connection time is reduced (in comparison to Bluetooth legacy) by dedicating three frequencies for discovery only, which means that there is less uncertainty for the paging device concerning where the other device is listening. In addition, since the frequencies are only used for discovering and not for the actual communication the probability that some other devices are using the same frequencies is minimized.
According to BLE terminology, a device which is available to set up a connection announces this by entering an advertising state where this availability is advertised. Another device may scan for, and receive, this advertising message. Upon reception of this advertising the device receiving the advertising message may in turn reply with a connection request in order to set up a connection to the device transmitting the advertising message. The time it takes for setting up a connection and transmitting one data packet may in this case be as little as 3 ms.
In addition, when several sensor devices already have established a connection to a gateway device, there is no time for connection set-up, but the time for the data transfer is determined by the transmission speed of the data. It may be beneficial to increase the data rate of BLE by adding a mode which supports at least twice the data rate. However, in order to be backwards compatible, the above disclosed advertising process should still be performed using the low rate mode.
Furthermore, currently several communications standards are able to operate in several modes, where the mode of operation may depend on the current channel conditions or it may depend on the ability of other devices. Bluetooth Enhanced Data Rate (EDR) supports up to 3 Mbps in a backwards compatible way. According to Bluetooth EDR two devices start out in the legacy 1 Mbps mode and then negotiate for the EDR mode. The possibility to use the EDR mode requires that both devices are EDR capable. If only one of the devices is EDR capable, the communication will have to use the legacy 1 Mbps mode.
Although this means that the more effective EDR mode can be used whenever both devices are EDR capable, it takes some time before the EDR mode may be entered since the mode has to be negotiated between the devices. For applications such as stereo audio streaming (which is a possible application for EDR), it does not matter if there is an initial negotiation phase which lasts 10-20 ms, when the actual application may run for several minutes. However, when the actual data transfer only requires a small amount of packets to be transmitted, as for example only one packet, starting out in one mode and later switching to a another mode may not be feasible.
Hence, there is still a need for an improved selection of which mode of operation to use for a communications session.